It's an evolving process and involves being able to let go of the very few items you carry on your back that you may become attached to or on the other spectrum despise because of the discomfort of the extra weight it adds.
I haven't gone a lot into description about my gear and/or specific weight items. But I will just a bit here. Starting out I wanted to have my base weight down to 18 pounds. I started at 22 pounds. That's four pounds to shed somewhere. Your base weight is things that you always carry with you, it's the constant that never changes. Unlike food, water, and other resupply items that fluctuate day to day. Also your total pack weight should never be heavier then a 1/3 of your total body weight. For me that's 46.5 pounds. Starting out on day 1 my pack fully loaded with 6 days of food and 20 miles worth of water (the first day is a bear with no water resupply for 20 miles) was 50 pounds. 3.5 pounds too heavy then I should ever carry.
Water weighs 2.2 pounds per liter on Day one I started with 5.5L that's 12 pounds of water. So you learn how to carry the bare minimum of water to make you comfortable and safe. For me that's 1L for every 5 miles in the desert and an extra 1.5L if I would be camping without a water source nearby. I've learned if I drink 1L for every 5miles I stay well hydrated and haven't had any bouts of dehydration yet. Also if it is super hot desert day and water is far between I may carry an extra .5-1L of electrolyte drink with me. To me downing a liter of electrolytes is like receiving a liter of normal saline (yes that's the nurse in me) and literally it perks me right up when feeling beat from the heat.
Food...when I resupply I eat my heaviest items first. Also still in the early stages of learning what I need, so far I made it to my resupply #1 with just the right amount. If I pick up mileage, eventually I may need less food, if I have less days between resupply. I love food it's my passion and I'm determined to have the best food on the trail...so no skimping here and no hungry bellys in my tent.
Now on to the big 3: shelter, sleep system, and pack, these are the 3 heaviest items you will carry, the should be a cumulative weight less than 9 pounds.
My pack: Dueter ACT Lite 60+10, 3lb 12oz
My sleep system: North Face Cat's Meow Women's Long, Sea-to-Summit silk liner and compression sack, 3lb
My tent: Big Agnes Fly Creek UL 2, 1lb 15oz (not counting tent stakes)
For a grand total of 8lb 11oz.
My pack is a little on the heavy side and my bag is on the heavier side as well, I'm happy with both right now, but if I need to replace either, I would look for a pack about a pound lighter and a bag about a pound lighter.
So what did a shed for weight my first go round? My Crocs, replaced by lighter weight cheap flop flops (thanks to Dale's suggestion at Mt Lagunas Outfitters), extra tent stakes and the tent stake bag, a bungee holding my thermalite sleeping pad together (I had 2 I got rid of 1), a 250g backup fuel canister, replaced by a 100g canister (I'm still on my 1st 100g canister and it's been 10 days) and last but not least one pair of underwear. (I did leave myself 1 pair-but honestly wearing underwear went out the door on day 2- sorry if it's TMI for some of you, but I'm sure lots of people-woman especially want to know). So everything individually weighs nothing, but add it all up and I probably got rid of just over a pound of weight. There are other things that once I run out of I won't replace, like my backup batteries for my headlamp, my facial sunscreen (just 1 will do the job) and once my feet toughen up I can lighten my 1st aid kit a bit. So that's what I got for round 1, I'll update you again once I let some more things go.